Combustion arrangements, hereinafter also referred to as burners, for solid fuel of the aforementioned type are known in various embodiments. Common to burners is the fact that they are suitably intended for fitting to some type of more or less conventional boiler, which suitably has a water-based heat supply system comprising the usual radiators, either as a complement to or as an alternative to the ordinary oil burner.
Some examples of these or essentially similar burners are disclosed by the patent publications WO 94/17331, WO 97/49951, SE-B-450 734 and GB-A-2 079 910, in which solid fuel burners are shown, which are fitted to a boiler so that the front section of the burner is introduced into the hearth of the boiler through the outer casing of the boiler. The said burners comprise a combustion chamber in which the solid and suitably granular fuel, in the form of pellets, for example, is rotated during introduction of the combustion air. Even larger solid fuel combustion plants are disclosed, for example, by the Swedish patent specification SE-C-63 193, which shows a furnace especially for the combustion of municipal refuse. The latter combustion arrangement also comprises a rotatable cylinder which functions as fuel grate.
In such combustion arrangements the fuel is therefore rotated with a simultaneous delivery of combustion air that contains the oxygen needed to bring about a primary combustion of the fuel. Fuel pellets normally consist of approximately 10% water and approximately 12% pure carbon, whilst the remainder of the pellets largely consists of various hydrocarbon compounds. The content of the pellets varies greatly, however. During the primary combustion hot combustion gases are formed on the one hand, together with ashes and other solid slag products on the other. The greater part, estimated at approximately 80 to 90%, of the ashes are entrained with the air flow through the burner as fly ash, which is precipitated out of the combustion gases outside the burner and inside the actual boiler. It is desirable for 100% of all ashes to be precipitated outside the burner, which normally occurs if the melting point of the ashes exceeds the temperature range at which the burner is intended to function, for example if the melting point of the ashes exceeds an operating temperature normally in the order of 1100° C., and if the air flow is sufficient. It has proved difficult, however, to obtain complete combustion of the fuel to secondary combustion gases, that is to say to achieve a fuel gasification of 100%. In combustion at temperatures in excess of the melting point of the ashes the original light, powdery ashes are in fact converted to pieces of fused heavier material, so-called sintering, which are not as readily entrained with the combustion gases out of the burner. In the combustion of impure fuel with excessively high contents of certain substances having inferior or deteriorating combustion characteristics the sintering also occurs at lower temperatures than the 1100° C. quoted, which further aggravates the problem of sintering.
In the burners currently known a substantial proportion of the sinter is therefore precipitated right in the actual combustion chamber, so that an accumulation of ashes, unburned pellets and sinter slag is formed, which obstructs the air inlet openings needed for the flow of air through the fuel bed into the combustion chamber. The obstruction of the air inlet openings results in impaired and uneven combustion of the fuel, so that more air must be added. This makes the burner less efficient, since the combustion gases are diluted and since the extra air delivered also has a cooling effect. The accumulation of ashes, pellets and slag grows quite rapidly to a greater height, which in turn can mean that the position of the fuel bed is shifted to a position that is not conducive to functioning, whilst the risk of burn-back also increases dramatically, that is to say the centre of the fire is raised towards and into the fuel feed pipe. This makes the sintering both technically awkward and moreover dangerous.
Non-rotating combustion chambers increase the aforementioned problems since the static nature of the combustion chamber means that the slag formation all the time occurs in the same area of the combustion chamber and since the automatic discharge normally performed in rotating combustion chambers by means of likewise rotating, screw-shaped discharge flanges is absent. Stationary combustion chambers therefore either require more frequent cleaning or a specially arranged cleaning device, such as an ash rake. In many boilers there is a container in the form of a box inside the boiler, in which box the ashes land since the front part of the burner is nested in the actual boiler. The ashes in the box are emptied either manually or by extracting the ashes by means of a suction device. The ash box may be relatively large. Therefore, it can be emptied relatively infrequently without causing difficulties.
In rotating combustion chambers the accumulation of ashes and sinter slag is therefore fed towards the outlet from the burner by means of similarly rotating discharge flanges. The accumulation also contains unburned pellets and other solid, not yet fully combusted products, however, which still have a substantial energy content. In order to also utilise the energy content of these products, the combustion chamber is therefore often designed with a convex longitudinal section by giving the walls of the combustion chamber a design diverging towards the front, open end of the burner. Alternatively, the burner may be provided with one or more edge flanges, which prevent the said products passing through the burner unburned. The patent publication WO 97/49951, for example, shows a burner having both an inner edge flange, which partially closes the outlet opening of the combustion chamber to a secondary combustion chamber arranged immediately outside this combustion chamber, and an outer annular edge flange, which partially closes the outlet opening of the secondary combustion chamber. In order to achieve combustion of the residual products in the secondary combustion chamber, there are secondary air inlet openings for secondary air arranged in the inner edge flange.
Since the partially closed construction of the burner not only prevents unburned residual products passing through the burner, but also impedes the flow of fly ash out of the combustion chamber, there is a greater risk that slag products will be formed inside the said combustion chamber and secondary combustion chamber at excessively high combustion temperatures. The secondary combustion chamber, moreover, entirely lacks any discharge flanges.
It will be appreciated therefore that one problem for solid fuel burners is the formation of sinter inside the actual combustion chamber and any secondary combustion chamber. It will furthermore be realised that in combustion arrangements with non-rotating combustion chamber without any automatic discharge of the slag products formed, in burners with combustion chambers having a convex longitudinal section or an outlet opening for the combustion gases which is smaller than the combustion chamber and/or the secondary combustion chamber itself the aforementioned problems increase very markedly.
There is therefore a desire for the burner to function over a longer period of time without special manual or automated burner cleaning measures. Instead, measures must be taken in the design of the burner in order to eliminate or at least substantially reduce the said sintering or to get the sintering of the ashes to occur at a safe distance outside the burner. Merely increasing the air flow by means of a larger blower, for example, in order to blow the ashes away might have undesirable effects on the fuel consumption, the efficiency and the temperature that are required in order to achieve an optimum operating cost.
A further problem is that the burner, and in the case of a rotating burner its bearing, may be damaged by excessively high temperatures. The specification GB-A-2 079 910 identifies this problem and states that the double-walled burner shown in the said specification has two purposes; firstly to deliver air to the combustion chamber and secondly to provide thermal insulation, that is to say air cooling of the combustion chamber bearings. The specification omits secondary combustion chambers.
In the case of existing burner design constructions, extensive and time-consuming work must be carried out in order to replace or repair a combustion chamber or secondary combustion chamber that has been burnt through. The main reason for the inside walls of combustion chambers and secondary combustion chambers becoming deformed and holes appearing in these is thought to be due to the fact that the flame jet generated by the burning combustion gases and the air delivered by the blower occurs at too short a distance from the said inside walls. One desire therefore is to be able to shift or definably limit the centre of combustion and hence the “volume” of the flame jet, that is to say the axial and radial temperature distribution of the flame jet from the said centre.
It will naturally be appreciated that even such a burner, that is to say a burner with a flame jet that can be controlled in the aforementioned way, has a limited life span, following which the combustion arrangement must dismantled so that combustion parts of the combustion arrangement, including at least the combustion chamber and secondary combustion chamber, must be replaced. Such replacement is costly and time-consuming since new parts still cannot be installed efficiently enough and since the replaced parts in the known design constructions represent an unnecessarily large part of the combustion arrangement.